The invention relates generally to a cooling system for a superconducting rotary machine, and in particular to a sealed thermal siphon cooling system and a method for cooling a superconducting rotor coil using such a system.
A superconductor is an element, inter-metallic alloy, or compound that will conduct electricity without resistance when cooled below a critical temperature. Superconductivity occurs in a wide variety of materials, including elements such as tin and aluminum, various metallic alloys, some heavily doped semiconductors, and certain ceramic compounds. In conventional superconductors, superconductivity is caused by a force of attraction between certain conduction electrons arising from the exchange of phonons, which causes the fluid of conduction electrons to exhibit a super fluid phase composed of correlated pairs of electrons.
Superconductors are useful in a variety of applications including magnetic resonance imaging systems and power generation and delivery systems, such as motors and generators. The loss of electrical resistance in the superconductor enables these devices to be operated with a much greater efficiency. High temperature superconducting coil field windings are formed of superconducting materials that can be brittle and must be cooled to a temperature at or below a critical temperature to achieve and maintain superconductivity.
Superconducting coils have been cooled by cryogenic fluids such as helium, neon, nitrogen, hydrogen and the like. However it is difficult to precisely control the amount of liquid cryogen surrounding the superconducting coil. If the amount of liquid cryogen surrounding the coil is too low or at an insufficiently low temperature, insufficient cooling of the coil occurs. If the amount of liquid cryogen surrounding the coil is too great or below a desired temperature range, and unevenly distributed around the coil, rotational imbalance may be generated in the machine. This can cause high radial vibration and potentially damage the machine, given the high rates of rotation and substantial radius in certain applications. Moreover, in existing cryogenic cooling systems for such machines, there is not sufficient storage of cryogen in the cooling system to allow service of the cooling unit without shut down of the electrical machine operation, resulting in loss of superconductivity and consequent complex shutdown and startup procedures.
Accordingly, there is a need for a technique that enables even distribution of liquid cryogen around a superconducting coil. In addition, a cooling system with reserve capacity that provides efficient cooling of a superconducting coil is also desirable.